Jerky is a food product that is manufactured under the guidelines of the USDA's public health agency, the Food Safety and Inspection Service (FSIS). It is manufactured for both human and animal consumption. The generally-accepted definition of jerky is a shelf stable (not requiring refrigeration), nutrient-dense, whole-muscle product. It can be marinated or not and sliced or ground. For manufacturing, it is normally formed from meat cut into strips or other shapes, and typically laid on screens and then heated to an internal temperature of greater than 160 degrees Fahrenheit for beef or greater than 165 degrees Fahrenheit for poultry. This heating generally occurs within an air stream having greater than 90% relative humidity.
Once the necessary internal temperature is achieved and the product is subject to the high relative humidity, the product is then dried in a conditioned airstream until a water activity of less than 0.70 is realized and greater than 7 log reduction of microbes is achieved. The most important microbes to be removed are Salmonella, E. coli O157:H7, and Listeria. If these steps (results) are not accomplished, then the product cannot legally be labeled jerky.
At present, the accepted way to manufacture jerky is to process it in thermal processing ovens called Smokehouses. The jerky is laid out on screens which normally are stacked on 4 inch vertical centers and attached to racking that is inserted into an oven. The oven has set point controls for both Dry Bulb and Wet Bulb temperatures, which are programmed to run for certain elapsed times in order to achieve the first step of heating and humidity application, which in turn achieves the greater than 7 log reduction of microbes. This step is the lethality step.
The next step is the drying phase where the Dry Bulb and Wet Bulb temperatures are again set with time restraints, this time to achieve a removal of free moisture (the water content) as well as the bound moisture which is a measure of the water activity. The correct water activity then being met verifies the process is complete and the product is now ‘Jerky’.
During the process, the airflow in the oven is achieved using convection that is produced by recirculating fans that discharge the air through nozzles or slots down into the oven after being conditioned with the controlled heat and humidity set points.
Drying the product using this method is by evaporation, using airflow across (around) the product to remove moisture that is exiting the body of the product. If the airstream cannot surround the product uniformly, the drying will be detrimentally affected, which leads to under or over drying and yield issues as well as food safety issues (Lethality).
This process can also cause case hardening of some surfaces, thereby trapping moisture. This often does occur in the majority of ovens because the air stream runs down the sides of the oven then returns upward, through the product, back to the fan. Since the jerky is laid flat, it impedes the airflow and non-uniform product is the result. To try to overcome this deficit, the process time has to be increased because the top screen of product is not as dry as the bottom screen of product and consequently the internal temperature varies, as does the yield.
Other issues occur during the initial heating step with the high relative humidity set point (the lethality step). The product, being colder than the airstream, has a lower dew point than the atmosphere and, therefore, condensation occurs on the product's surface creating more moisture to be removed, as well as discoloration (blanching). In order to dry the product, fresh air is injected into the oven and is then exhausted, which carries out excess humidity (moisture). This process also is in the presence of oxygen (which is a requirement of bacteria and mold growth) adding to the safety issues.
These issues increase the drying process and extend the total processing time by approximately 25%. The average process takes approximately 6 to 10 hours or more to complete. This depends on the thickness of the product and the efficiency of the ovens used.
A few prior art references are noted. U.S. Pat. No. 4,495,932 is directed to “freshen” precooked foods using steam for heating, then the steam is dried and superheated and flows around the food compartment imparting a limited amount of moisture to freshen it. This patent uses superheated steam to heat the product and add moisture. In reality, the dry superheated steam removes moisture from the product, which is the case in our invention.
U.S. Pat. No. 8,044,326 is directed to home use steam cookers involving a vapor heating heater. This patent uses superheated steam to heat a chamber which then is used as a radiant heater. We do not use this technology. In the present invention, the pre-heating of the chamber is done by a separate heat source with convection and then super-heated steam is added. This is done to eliminate any condensation occurring within the chamber.
U.S. Pat. No. 5,505,975 is directed to cooking under pressure with compressed air and superheated steam. This also differs from the present invention, which does not use pressure in the chamber. In the present invention, steam is pressurized into the chamber but the chamber is open by exhaust and therefore not pressurized. Compressed air is not used. The use of compressed air would condense the steam since compressed air temperature is below that of the saturation temperature of the steam.